Context. Absorption or emission lines of Fe ii are observed in many astrophysical spectra and accurate atomic data are required to interpret these lines. The calculation of electron-impact excitation rates for transitions among even the lowest lying levels of Fe ii is a formidable task for theoreticians.Aims. In this paper, we present collision strengths and effective collision strengths for electron-impact excitation of Fe ii for low-lying forbidden transitions among the lowest 16 fine-structure levels arising from the four LS states 3d 6 4s 6 D e , 3d 7 4 F e , 3d 6 4s 4 D e , and 3d 7 4 P e . The effective collision strengths are calculated for a wide range of electron temperatures of astrophysical importance from 30-100 000 K. Methods. The parallel suite of Breit-Pauli codes are utilised to compute the collision cross sections for electron-impact excitation of Fe ii and relativistic terms are included explicitly in both the target and the scattering approximation. 100 LS or 262-jj levels formed from the basis configurations 3d 6 4s, 3d 7 , and 3d 6 4p were included in the wavefunction representation of the target, including all doublet, quartet, and sextet terms. Collision strengths for a total of 34 191 individual transitions were computed. Results. A detailed comparison is made with previous theoretical works and significant differences were found to occur in the effective collision strengths, particularly at low temperatures.

The charge state distributions of Fe, Na, and F are determined in a photoionized laboratory plasma using high resolution x-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter xi=20-25 erg cm s(-1) under near steady-state conditions. Line opacities are well fitted by a curve-of-growth analysis which includes the effects of velocity gradients in a one-dimensional expanding plasma. First comparisons of the measured charge state distributions with x-ray photoionization models show reasonable agreement.

We extend the range of validity of the artis 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium (non-LTE) population and ionisation solver, a new multi-frequency radiation field model, and a new atomic dataset with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionisation, and heating. We validate our method with a variety of tests including comparing our synthetic nebular spectra for the well-known onedimensional W7 model with the results of other studies. As an illustrative application of the code, we present synthetic nebular spectra for the detonation of a sub-Chandrasekhar white dwarf in which the possible effects of gravitational settling of 22 Ne prior to explosion have been explored. Specifically, we compare synthetic nebular spectra for a 1.06 M white dwarf model obtained when 5.5 Gyr of very-efficient settling is assumed to a similar model without settling. We find that this degree of 22 Ne settling has only a modest effect on the resulting nebular spectra due to increased 58 Ni abundance. Due to the high ionisation in sub-Chandrasekhar models, the nebular [Ni ii] emission remains negligible, while the [Ni iii] line strengths are increased and the overall ionisation balance is slightly lowered in the model with 22 Ne settling.In common with previous studies of sub-Chandrasekhar models at nebular epochs, these models overproduce [Fe iii] emission relative to [Fe ii] in comparison to observations of normal Type Ia supernovae.A natural candidate for triggering the ignition is for the WD to

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Effective collision strengths for electron-impact excitation of Fe II are presented for all sextet-to-quartet transitions among the 38 LS states formed from the basis configurations 3d64s, 3d7 and 3d64p. A total of 112 individual transitions are considered at electron temperatures in the range 30–100 000 K, encompassing values of importance for applications in astrophysics as well as laboratory plasmas. A limited comparison is made with earlier theoretical work and large differences are found to occur at the temperatures considered. In particular, it is found that the inclusion or omission of some (N + 1)-bound configurations in the Hamiltonian matrices describing the collision process can have a huge effect on the resulting effective collision strengths, by up to a factor of four in some cases.

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Partial wave collision strengths are presented for low-energy electronimpact transitions in Fe II between the 3d 6 4s a 6 D e ground state and the 3d 7 a 4 F e , 3d 6 4s a 4 D e and 3d 7 a 4 P e low-lying excited states. The collision strengths are calculated in LS coupling using a new Fortran 95 R-matrix program including all terms of the 3d 6 4s, 3d 7 , 3d 6 4p, 3d 5 4s 2 and 3d 5 4s4p configurations in the close coupling expansion of the collision wavefunction. Special emphasis is given to the inclusion of configuration interaction (CI) effects both in the target and in the collision wavefunctions. In both cases series of calculations are carried out where additional CI terms are included systematically. It is found that in order to obtain close to converged low-energy partial wave collision strengths two-electron excitations from the 3p shell to the 3d shell as well as pseudo s and d orbitals must be included in the CI expansions. Also resonance effects in low-energy partial wave collision strengths are found to depend sensitively on the representation of the d orbitals in the CI expansion of the collision wavefunction. Finally, CI models for both the target and collision wavefunctions are defined which can be used in proposed calculations to obtain accurate total collision strengths and effective collision strengths for transitions between LS-coupled terms and between fine-structure levels of Fe II.

We use a simple average-atom model (NIMP) to calculate the distribution of ionization in a photoionization-dominated plasma, for comparison with recent experimental measurements undertaken on the Z-machine at the Sandia National Laboratory. The agreement between theory and experiment is found to be as good for calculations with an average-atom model as for those generated by more detailed models.